In order to produce, for example, block copolymers, there has conventionally been known a method for producing a polymer where ring-opening polymerizable monomers are polymerized at a plurality of divided stages. In one exemplary disclosed method, D-lactide is polymerized to obtain poly-D-lactic acid, and the poly-D-lactic acid is polymerized with L-lactide, to thereby produce a stereo block copolymer of polylactic acid (see PTL 1). In accordance with the disclosed method, dichloromethane which is an organic solvent is used in each of the step of polymerizing D-lactide and the step of polymerizing L-lactide. When polymerization is performed using an organic solvent, there is a problem in that the organic solvent remains as a waste liquid after the polymerization.
As for a method for polymerizing a ring-opening polymerizable monomer without using an organic solvent, there is disclosed a method for polymerizing lactide in supercritical carbon dioxide (see NPL 1). In accordance with the disclosed method, lactide is polymerized in a procedure including: charging an autoclave with lactide, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and benzyl alcohol; adding carbon dioxide to the autoclave; stirring the mixture at 80° C. and 70 atm; and adding carbon dioxide to the autoclave to turn the pressure to 250 atm. In accordance with this method, a polymer having a number average molecular weight of about 10,000 is obtained by reacting for 16 hours.